All fencing and other safeguards provided in pursuance of sections 38 and 39 shall be of substantial construction, and constantly maintained and kept in position while the parts required to be fenced or safeguarded are in motion or in use, except when any such parts are necessarily exposed for examination and for any lubrication or adjustment shown by such examination to be immediately necessary, and all the conditions specified in the relevant safety regulations made under section 51 are complied with.